1. Field of the Invention
The present invention relates to a magnetic disk comprising a substrate, and the use of glasses having a high specific modulus of elasticity for the manufacture of hard disk substrates, for example, substrates for hard disk drives in computers.
2. Background Information
Glass has advantages over metals such as aluminum or metal alloys for use as a substrate for data media (hard disks), among other things on account of its low surface roughness and its flatness. Such substrate glasses must be able to withstand the increased chemical, thermal and mechanical loads experienced during use. For example, during coating (e.g. by cathodic sputtering), they are exposed to high temperatures and rapid cooling rates. When they are used as hard disks, they are also exposed to high mechanical loads, e.g. during installation on the drive shaft they experience loads of up to 100 N/mm.sup.2, and in operation, they can experience additional stresses caused by centrifugal forces at high speeds of currently 3,500 to 10,000 rpm. Such loads can be survived by thin glass, which is generally 0.25 to 3.0 mm thick, generally only if it has been surface-tempered. Because the permissible mechanical load carrying capacity can be increased by thermal tempering only above a minimum thickness of 3 mm, it must be possible to chemically temper the glasses used for the above mentioned application. Accordingly, the glass can be tempered by ion exchange in a salt bath below the transformation temperature T.sub.g, i.e. the glass must have enough ions such as Li.sup.+ and/or Na.sup.+ suitable for the exchange. In addition to the surface flatness, the chemical resistance of the substrate glass is of decisive importance for the functionality of a fixed disk, because the read-write head is currently supported on a cushion of air at a distance of only about 50 nm above the rotating hard disk. This distance must remain substantially guaranteed for correct operation. This distance is reduced, however, if the surface of the hard disk substrate is not resistant to atmospheric effects and/or if chemical corrosion has roughened the surface even before the coating has been applied, or if the surface loses its adherence to the applied coatings on account of atmospheric influences, and the coating subsequently detaches from it. An additional essential characteristic of glasses that are suitable for use as hard disk substrates is their thermal expansion behavior, which must not be too different from that of the coating materials (e.g. co-alloys that have coefficients of thermal expansion .alpha..sub.20/300.gtoreq.12.times.10.sup.-6 /K) and above all not too different from that of the clamping materials and spindle materials of the disk drive (at .alpha..sub.20/300.gtoreq.12.times.10.sup.-6 /K), to eliminate stresses.
One requirement to increase the density of information and the read-write speed is a reduction of the floating height of the read-write head above the hard disk. A lower head gap or floating height makes faster access times possible.
The floating or flying height cannot currently be reduced to any desired height, because when the hard disk is rotating, fluctuations in the drive system can be caused by locally severe air turbulence and by impacts, that are expressed in a sort of fluttering movement of the hard disk. If the floating or flying height of the read-write head were too low, these deviations from the neutral position would cause the read-write head to lose its orientation to the information content of the spot on the hard disk ("runout"), or the read-write head might even collide with the hard disk ("head crash").
To prevent these undesirable occurrences and to make possible high speeds of rotation, the hard discs substantially require a high dimensional stability, i.e. they should have the smallest possible flutter on their outer edges as a function of time.
The maximum disc flutter W is described by the following formula: ##EQU1##
where: ##EQU2##
On the basis of this formula, the principal requirement for new materials for hard disks is that they reduce the maximum flutter W with a high modulus elasticity E and/or a low density .rho., with uniform geometry (r.sub.A, d constant). Conventionally, the quotient of these two parameters E/.rho. is designated the specific modulus of elasticity. Like the desired high modulus of elasticity, this parameter should assume the highest possible value.
As a material for high speeds of revolution, there is a composite material consisting of Al-B-C (IDEMA, Alternative Substrates III (Sep. 5, 1995, San Jose, Calif.), p. 55-60: D. J. Perettie et al. "The Alternate Alternative Substrate--"Chemically Strengthened Aluminum"), which has a low density, high strength and a very high specific modulus of elasticity E/.rho.. However, the above mentioned material is extremely difficult and expensive to polish to the surface quality with a roughness (RMS value) of less than 0.4 nm required for high-quality hard disks. It is very expensive to manufacture hard disks from this material, primarily on account of the high abrasion hardness.
WO 96/04651 describes a data medium in the form of a composite disk made of glass and a visco-elastic material, in which vibrations are damped by the layer of the visco-elastic material, which can, for example, be made of plastics such as synthetic rubbers, e.g. silicon rubber, or polyester, polyurethane or polyamide. One disadvantage of this method is that the manufacturing process is very expensive, and the visco-elastic material experiences fatigue (becomes brittle) with time, and can no longer function as a vibration damper. The plastics used can also be degasified if the magnetic layer is deposited by cathodic sputtering at an elevated substrate temperature, which thereby has an adverse effect on the quality of the layer applied.